Add support for extension based config item

[milanc]

This is to address feature request I've created #96.

Behavior

Once extension is added to config file then all the files with that extension are compiled without need for adding them to the config.
Right click on the file with a configured extension will not have option to be removed from the compiler config.
Recompile all will also compile all files matching the extension.
Running the compilation from Task Explorer will compile all files matching the extension.
To avoid performance impact, folder scan for files matching extension is cached.
New files, added through visual studio, that match the extension will also be compiled.

Known Issue

If files are added outside visual studio while the project is open, we need to run recompile all for the file to be included in compilation.

With these changes the extension supports config file like this.

[
  {
    "outputFile": "*.razor.css",
    "inputFile": "*.razor.scss",
    "minify": {
      "enabled": false
    }
  },
  {
    "outputFile": "stest.css",
    "inputFile": "stest.scss"
  }
]

And task explorer shows this

[mstancombe]

Could this instead just search anywhere for the wildcard character? Just thinking of cases like /MyFolder/*.less